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INSTRUCTION SET REFERENCE
PSUBB/PSUBW/PSUBDPacked Subtract (Continued)
The PSUBD instruction subtracts the doublewords of the source operand from the doublewords
of the destination operand and stores the results to the destination operand. When an individual
result is too large to be represented in 32 bits, the lower 32 bits of the result are written to the
destination operand and therefore the result wraps around.
Note that like the integer SUB instruction, the PSUBB, PSUBW, and PSUBD instructions can
operate on either unsigned or signed (twos complement notation) packed integers. Unlike the
integer instructions, none of the MMX instructions affect the EFLAGS register. With
MMX instructions, there are no carry or overflow flags to indicate when overflow has
occurred, so the software must control the range of values or else use the with saturation
MMX instructions.
Operation
IF instruction is PSUBB
THEN
DEST(7..0)
<
DEST(7..0) SRC(7..0);
DEST(15..8)
<
DEST(15..8) SRC(15..8);
DEST(23..16)
<
DEST(23..16) SRC(23..16);
DEST(31..24)
<
DEST(31..24) SRC(31..24);
DEST(39..32)
<
DEST(39..32) SRC(39..32);
DEST(47..40)
<
DEST(47..40) SRC(47..40);
DEST(55..48)
<
DEST(55..48) SRC(55..48);
DEST(63..56)
<
DEST(63..56) SRC(63..56);
ELSEIF instruction is PSUBW
THEN
DEST(15..0)
<
DEST(15..0) SRC(15..0);
DEST(31..16)
<
DEST(31..16) SRC(31..16);
DEST(47..32)
<
DEST(47..32) SRC(47..32);
DEST(63..48)
<
DEST(63..48) SRC(63..48);
ELSE { (* instruction is PSUBD *)
DEST(31..0)
<
DEST(31..0) SRC(31..0);
DEST(63..32)
<
DEST(63..32) SRC(63..32);
FI;
